A captured image can be viewed three dimensionally by binocular stereopsis if a distance from an imaging device such as a camera to a subject of the captured image is known. This can be made by generating two images that are shifted from each other to an extent corresponding to a binocular disparity based on the distance and by observing the images with the right eye and the left eye, respectively. In order to implement such a system, however, it is necessary to provide a display configured to divide pictures for the right eye and the left eye spatially or temporally and display the divided pictures, for example. In addition, the configuration is complex since a viewer has to wear glasses for the system, for example. Thus, if pictures can be displayed two-dimensionally with an enhanced three-dimensional appearance, simplified stereoscopic pictures can be viewed, which is advantageous to viewers.
Enhancement of shades of a subject increases uneven impression thereof and produces an effect of enhancing the three-dimensional appearance of the image. Uneven impression is defined herein as expressiveness of the unevenness of a subject surface. A technique of enhancing the gradation of an image, which is one element to express shades, is known as a technique for producing uneven impression. In this technique, a gradient of an image is calculated and a component with a gradient whose magnitude is small is enhanced to produce the effect.
In the conventional technology, however, the shades are enhanced uniformly on the entire screen. Thus, the uneven impression of a distant background, for example, is also increased. A subject with increased uneven impression is perceived to be nearer than that before being processed. Therefore, there has been a disadvantage that the distance between the foreground and the background is perceived to be small, which results in an image having a flat composition with a low depth feel. A depth feel is defined herein as expressiveness of a distance in the depth direction between subjects. Uneven impression and a depth feel are both considered to be main components providing a three-dimensional appearance of an image.